The present invention relates to a printed circuit board.
A conventional printed circuit is shown in FIG. 3, and is formed on a copper clad laminated plate by etching the copper film on the plate to form a desired circuit conductor 2 on a base plate 1.
To shield outer noise from reaching the circuit conductors 2 on the printed circuit board 3, before desired instruments are attached to on the printed circuit board 3. Shield film 4 having a shielding function is placed on the board 3 to locally shield a portion or portions where outer noise is to be prevented from reaching circuit components. In FIG. 3, 5 designates an insulation layer which overlays the circuit conductors 2.
Conventionally, the shield film 4 on the printed circuit board 3 is made as a separate member, and is cut to desired dimension before the film is placed on the printed circuit board 3. The applicant of the present invention proposes an improved printed circuit board shown in FIG. 4, having a shield layer which can be formed industrially in the course of making the printed circuit board.
The known printed circuit board 6 shown in FIG. 4 is formed such that after the circuit conductor 2 is formed on the base plate 1, an insulating layer 5 is formed, and on the insulating layer 5 paste having a shielding function is printed by a silk screen printing process, and then the paste is hardened to form shield layer 7. Then, an insulating layer 8 is formed on the shield film 7.
The printed circuit boards 3 and 6 are each formed by forming shield film 4 or shield layer 7 on the circuit conductor 2 so that outer noise can be shielded from the conductor 2. However, cross talk or noise effect between the conductors 2 can not be effectively shielded.
The present invention aims to eliminate such drawbacks and to provide an improved printed circuit board having no cross talk and noise effect between conductors and from the outside environment.